| Space laser communication refers to the communication technology that uses optical carrier to transmit information in the free space,which has the advantages of strong directivity,anti-electromagnetic interference ability and rich frequency resources.So,it has gradually become the focus of research in the field of optical communication,and has been widely used in satellite-to-ground communication,ground communication and military fields.However,the communication link of space laser communication is difficult to relay.In order to give full play to its advantages of high speed and large capacity,the modulation format is required to have the characteristics of high frequency spectral efficiency and high power efficiency.Therefore,it is an important work to study the modulation technology suitable for space laser communication.In addition,atmospheric turbulence will lead to the random fluctuation of the intensity and phase of the propagating beam,resulting in the deep fading of the received signal and the degradation of the performance of the space laser communication system.Therefore,the research on the receiving technology that can overcome the influence of atmospheric turbulence also has important application value and significance.This paper focuses on the space laser communication system and investigates the modulation technology and reception technology,respectiviely.Based on the in-depth analysis of the impact of the modulation format on the performance of the communication system,a four-dimensional modulation format with high sensitivity is designed.Based on the analysis of the efficient free-space optical reception capability of few-mode fiber,the few-mode heterodyne detection technology and mode diversity reception technology are deeply studied and implemented,respectively.The main contributions and the innovation points of this dissertation are as follows:1.Aiming at the requirements of high spectral efficiency and high power efficiency modulation format for space laser communication,the performance of hybrid modulation for space laser communication is studied and analyzed,and a high-performance four-dimensional modulation format is proposed..Firstly,a space optical communication experimental platform in laboratory based on modified Von Karman model turbulence is built.Using this platform,the transmission of 10 Gbps mPPM-QPSK mixed modulation signals under different turbulence is realized.The effect of atmospheric turbulence intensity on the bit error rate of mixed modulation format is studied experimentally.Secondly,a four-dimensional modulation format4D-nMD is proposed to meet the demand of high frequency spectral efficiency.The modulation format selects constellation points in the four-dimensional space,and realizes the mapping from multiple bit labels to four-dimensional amplitude through Gallager mapping.Finally,the optical simulation software optisystem is used to build a space optical communication system,and the performance of the proposed modulation format and several modulation formats under the same spectral efficiency are verified and compared.2.In order to solve the problem that heterodyne detection based on single-mode optical fiber reception is seriously affected by atmospheric turbulence,a few-mode heterodyne detection and reception technique with turbulence compensation capability is proposed.Firstly,the mathematical model of few mode heterodyne detection based on few mode fiber mixing is established,and the performance of few mode heterodyne receiver under different turbulence conditions is analyzed by numerical simulation data.Then,aiming at the problem that heterodyne detection requires strict frequency and phase matching or locking between local oscillator light and signal light,a few mode heterodyne detection method based on Kramers Kronig relationship is proposed.Finally,a space optical communication system based on few mode heterodyne detection is built in the laboratory.On the one hand,the coupling characteristics of few mode fiber and single-mode fiber in atmospheric turbulence channel are measured;On the other hand,the few mode heterodyne detection reception of 4Gbps QPSK information is realized.The experimental results show that under medium and strong turbulence conditions,when the bit error rate reaches at forward error correction threshold,the performance of few mode heterodyne detection is about 5dB higher than that of heterodyne detection received with a single-mode fiber.The designed system can well compensate the medium and strong turbulence effect.3.Mode diversity is an emerging reception technique that uses few-mode fiber to compensate for the atmospheric turbulence effects,but has very complex electrical domain structure which leading to expensive cost in implement.Considering this problem,a mode diversity reception based on non-mode selective photon lantern and equal gain combining(EGC)is proposed and implemented,which has high performance gain and low complexity.Firstly,the space optical communication system based on mode diversity reception is mathematically modeled.For EGC method,an index to measure the difference between the mode diversity reception system and the theoretical performance upper limit is proposed and verified by experimental measurements.Then,the space optical communication system based on mode diversity reception in the laboratory is built with the mode-selective photon lantern and non-mode-selective photon lanterns.And the power distribution characteristics of mode diversity reception of different photon lanterns are measured and analyzed.Finally,the mode diversity reception of 4Gbps QPSK signal is realized,and the average bit error rate and interrupt probability of the system are measured.The experimental results show that the performance differenc between the scheme with the non-mode selective photon lantern and EGC and the upper performance bound is less than 1dB under weak to strong turbulence,and is 1.9dB better than the scheme of mode selective photon lantern with EGC.It is the best choice for mode diversity EGC scheme. |
PDF Full Text Request